Quantitation of RET Efficiencies in Proteins

In the previous secdon we explained the emissiem ectra of intnferon Y in terms of energy transfer, but we did not consid the effect of dim dissociation on the fluorescence intensity of the tryptophan residue. How could we kn that the tryptophan quantum yield did not decrease on idissociation of interferDn y into monomers In fact, the tryptophan emission intensity decreases ai noximately twofold on dissociation. Fortunately, dieie is a means to measure the efficioicy of tyr-to-ttp enragy transfer wdiidi is independent of Ute tryptophan quantum yield. 

The effideo of RET in proteins can be measured by cemsidering die rdative absoihance of die aromadc andno 

This concept of excitadon-dependent quantum yields is shown in I ure 16.19 for an eqmmolar nnxture and a 2 1 1 mixtiHe of tyrodne, tf t(q)lm, and phnnylalaniiie. The upper curves represent die fraction of the total light ahsort Ity tryptophan. The fractional disofbance due to tryptophan is given by 

RET can also occur from phenyldanine to tyrosine. This was found in an unusual hlstbifelike protein, HTa, fiom a thennopfailic archaebacterium Thermoplasma acidophi lum. associates strongly with DNA to protect it 

F ure16.24. Ultraviotecabsofption spectra of Ibehisiiyi kBiHOtein HTa. (A) Native protein HTa (B) mixtare of aromatic amino 

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